In Situ Monitoring of the Nucleation of Polyaniline Nanoparticlesfrom Sodium Dodecyl Sulfate Micelles: A Nuclear MagneticResonance StudyXiaodong Wu,*,† Kong Liu,† Lude Lu,† Qiaofeng Han,† Fengli Bei,† Xujie Yang,† Xin Wang,† Qiong Wu,‡

and Weihua Zhu‡

†Key Laboratory for Soft Chemistry and Functional Materials, and ‡Institute for Computation in Molecular and Materials Science andDepartment of Chemistry, Nanjing University of Science and Technology, Nanjing 210094, People’s Republic of China

*S Supporting Information

ABSTRACT: A complete mechanistic study on the nucleation ofpolymeric nanoparticles covering the generation of the clusters and theforthcoming aggregation to the nuclei is performed by in situ 1H nuclearmagnetic resonance (NMR) experiments using polyaniline as anexample. An aniline tetramer and a monomer-stabilized nitrenium areproved to be the basic cluster and predominant propagating intermediatein the nucleation. It is observed that the nuclei are generated via asequential mechanism involving a translocation of the protonatedtetramers to the aqueous bulk, dissociation of sodium dodecyl sulfate(SDS) micelles, and deprotonation to induce the fusion of thedissociated micelles and intermolecular packing of the oligomers. Despiteits importance, direct observation of the nucleation is challenging. Thiswork emphasizes the importance of utilizing the solvent in solvation shellas the sensitive probe to explore the most transient process in nucleation, demonstrating its efficiency in achieving informationsuch as the stepwise procedures, the nuclei sizes, the growth kinetics, and so forth. The approach reported herein may prove ofgreat value in establishing the missing link of the atomic origin of nanoparticles, a key topic toward the preparation of functionalnanostructures with well-controlled architectures, and can be readily extended to the study of other organic or inorganic systems.

■ INTRODUCTION

Nucleation, a common phenomenon throughout nature andtechnology, is defined as the series of atomic or molecularprocesses by which the atoms or molecules of a reactant phaserearrange into a critical nucleus of the product phase largeenough as to have the ability to grow irreversibly to amacroscopically larger size.1−4 The most significant feature ofthis process is the creation of the nascent nuclei, whichdetermines the forthcoming growth pathways and theaccompanied kinetics, yielding a product with well-definedphase, structure, and morphology.5−9 Therefore, it is thecornerstone of many important modern technologies, rangingfrom the inanimate fields such as the material design,mechanical engineering, semiconductor/IT, and nanotechnol-ogy to the life science and genetic engineering.10−16

Up to now, significant progress has been achieved in themechanistic study on nucleation in gaseous phase, andtheorems such as VLS (vapor−liquid−solid) have been putforward, providing enhanced possibility to manipulate thestructural-transformation dynamics at atomic scale.17,18 In greatcontrast, the corresponding work in solution is still of greatscientific challenge. Nevertheless, most reactions take place insolution, so this knowledge is a requisite for chemists toprecisely design, control, and grow the desired structures in

practice. Now theoretical simulations have provided a wealth ofinformation on the structural and dynamic properties of thenascent clusters, taking place during periods difficult to beexplored experimentally,19,20 whereas direct microscopic orspectroscopic observations on the structural changes from thenascent nuclei in the course of the reaction have enriched ourknowledge on their growth behavior.21−23 Meanwhile, in situscattering techniques have been applied toward understandingthe temporal evolution of the templates.24−26 Cumbersomely,very little information is known regarding the transition fromthe clusters into the nuclei, the atomistic origin of the wholeprocess. The uncertainty of this has eluded us to decipher thewhole nucleation scenario.The nucleation phenomenon is transient and extremely

sensitive to external disturbances. Picking up such rapidlydecayed signals from the immense background noise in mostcases is a mission impossible, and therefore no approach hasbeen developed so far. To address this issue, we believe shiftingour research focal point is required. While considerableattention has been paid on the clusters, the nuclei, and the

Received: December 28, 2012Revised: April 10, 2013Published: April 11, 2013

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reaction environment such as the templates, little systematicdesign and operation has been performed on the solventmolecules, in particular the ones in the solvation shell.Discarding the information inherent in these moleculesmeans losing one of the most important clues for the resolutionof the whole process. As in colloidal directed reactions, onemost popular method in generating well-defined structures insolution, water in hydration shell is in close contact with bothreactants and templates to form an integral part of the moleculein question, and as such it may potentially response and evendictate key structural transitions in nucleation.27,28

In theory, these molecules are the most active and sensitiveprobe for the study.29 At the beginning of the nucleation, theirtotal number is hundreds and thousands times larger than thatof the nuclei, which guarantees the high sensitivity of themeasurement.30 Second, they are located at the outermost ofeach phase and are very mobile, and thus respond to anychemical or physical changes very fast, making the detectionfeasible in the time scale.31 Finally, modulation of their mobilityas determined by the bounding degrees allows one todifferentiate them from the bulk part, potentially favorable forhigh-resolution detections.32 While superior in nature, thesemolecules are of limited use due to their rapid exchange withthe bulk one, leading to the desired signal decaying too fast toyield an averaged one.33 Considering any phase transition willarouse an instantaneous spatial reorganization of all compo-nents; such physical violence functions the same as mechanicalstirring, accelerating the solvent exchange while burying thewanted signals into noisy background. However, it can beenvisioned that an initial system with a large body-size will havemore power to withstand such instantaneous shocks,prolonging the response time and more importantly slowingthe solvent exchange. In this way, interested signals from thesolvent that carries the information for the phase transition,although feeble, may be detectable. Moreover, this systemshould be at best in its thermodynamically steady state becauseminimization of the internal energy to its lowest implies thatmost species detected be in a nearly identical condition,permitting maximization of the intended weak interactions,minimization of side reactions, and a relatively uniquenucleation style.34

Technically, nuclear magnetic resonance (NMR) spectros-copy is a powerful tool to address this transition because (i) anyspecies within the size of interest is still more or less mobile andexhibits reasonable solubility, beneficial for obtaining high-resolution spectra. Even in some special cases the resulting line-width is broadened, but such a broadening provides valuableinformation on the residual dipolar or quadrupolar interactionsas well as the molecular dynamics of the studied species.35−37

(ii) Once any new phase is formed upon nucleation, thephysical properties such as the bulk magnetic susceptibilities ofthe same compound can be different in these two phases,facilitating to differentiate one from another.38,39

Polyaniline (PANI) has demonstrated its superiority asconducting polymers due to its mild reaction condition in thesynthesis, excellent chemical stability, appropriate conductivity,and unique proton doping−dedoping properties, and thereforehas widespread applications in fields such as light emittingdiodes, organic solar cells, gas sensors, and so forth.40−42 Theupcoming of the nanostructured products as well as theirnanocomposites further stimulates its development, and thus alarge number of polymers with well-controlled morphologieshave been prepared.43,44 Impacts from the oligomers as well as

the optimization of experimental parameters have beenperformed to reveal their formation mechanism.45−50 Yet,despite its importance, no definite conclusions on the detailedatomistic origin have been drawn so far. Herein, we have grownsodium dodecyl sulfate (SDS) micelles into larger sizes prior toany measurement. In this way, the water in hydration shellbecomes the most sensitive signal and enables us to have adirect observation of the detailed nucleation processes,specifically how aniline tetramers, the clusters, are aggregatedinto the nascent nuclei. To the best of our knowledge, this isthe first time to catch up this fleeting process.

■ EXPERIMENTAL SECTION

Materials and Methods. Materials. Sodium dodecylsulfate (99%) was purchased from Alfa Aesar and used asreceived. Aniline and ammonium persulfate (APS) wereobtained from Sinopharm Chemical Reagent Co. Ltd. Anilinewas distilled under reduced pressure before usage. Deuteriumoxide and 1,4-dioxane were purchased from Beijing ChemicalReagents Co. Ltd.

Stock Solutions for NMR Measurements. A series ofaniline/SDS stock solutions were prepared. In each measure-ment, the concentration of aniline was fixed to be 0.086 M,while that of SDS changed in the order of 0.005, 0.008, 0.04,0.06, and 0.10 M. Both compounds were mixed well in a D2Osolution with 0.05% of trimethylsilyl propanoic acid (TMSP).

Synthesis of Polyaniline. The monomer was assembled withSDS for at least 2 days prior to initiating the reaction by APS.All other procedures followed the same steps according to theliterature.47

Instrumentation. High-resolution transmission electronmicroscopic (HRTEM) images were obtained on a JEM-2100microscope (JEOL).

NMR Measurements. Before any NMR measurement, a totalvolume of about 0.5 mL of the stock solution was first filled inan NMR tube and kept at 25 °C for 2 days. All NMRexperiments were performed on a Bruker DRX 300spectrometer (300.13 MHz for 1H), equipped with a BBIgradient probe with a maximum gradient strength of 52.7 Gcm−1. The measurements were performed at 25 °C, and thetemperature was controlled to within ± 0.1 °C with a BrukerDVT300 digital controller. The in situ experiment was designedas such the uniformly dispersed aniline in different SDSmicelles was polymerized in an NMR tube by adding APS, anda series of 1H NMR spectra were collected as a function of thereaction time. 1H NMR diffusion measurements wereperformed using the pulsed-field gradient (PFG) stimulated-echo (STE) procedure,51,52 with phase cycling of the radiofrequency pulses to remove unwanted echoes.53 The gradientstrength was calibrated from the known diffusion coefficient ofHDO at 25 °C.54 Typical parameters used in our experimentswere a total diffusion encoding pulse duration δ of 3.5 ms, and adiffusion delay Δ of 20 ms. The diffusion coefficient wasyielded by monoexponential fitting of the echo intensities as afunction of the echo time using the Bruker Topspin 1.3software package.

■ RESULTS AND DISCUSSION

When the synthesis of nanoparticles is carried out within themicelles, solubilization locus, that is, the location of thereactants in the micelles, is one of the most important factors,which significantly affects the nature of the reaction, the

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reaction kinetics, and thereafter impacts strongly on theformation of the cluster for the nascent embryo in nucleation.Hence, a comprehensive understanding of the status of bothaniline and SDS prior to polymerization is a prerequisite. Tothis end, the highly sensitive chemical shifts in 1H NMR areutilized to determine the locus,55,56 while the mobility of eachcompound is assessed by their diffusion coefficients measuredthrough the diffusion ordered spectroscopy (DOSY).51 Ourchemical shift analysis from anilines suggests that they arelocated at the hydrophilic domains in SDS spherical micelles,whereas in the hydrophobic cores when they are in rod-likeones. Further analysis of the resonance from γ-methylenegroups as induced by ring current shift effects from aniline57,58

suggests a decreased relative concentration ratio of aniline overSDS in rod-like systems. The diffusion coefficients of aniline inthe presence and absence of surfactant shown in Table S2 helpus to assign most anilines locating in the Gouy−Chapman layerin 0.005 M SDS micelle while in the Stern layer in 0.008 M.(See the Supporting Information for detailed spectral analysis.)Collectively, all deduced information is summarized in Figure 1.The differences between each state include the aggregationstyle of SDS, the location and the relative concentration ofaniline, as well as the mobility of each species.

To explore the nucleation mechanism, two basic questionshave to be answered: (1) how the clusters, the monomers forthe nascent nuclei, are produced and (2) how they areaggregated into the nucleus center. In this section, systematic insitu NMR experiments were performed to reveal the formationmechanism of the clusters. It should be mentioned that,although a number of mechanistic studies on polyaniline insolution have been carried out,59−62 the corresponding work inmicelles is limited. Because of the localized monomer and thepotentially stabilized intermediate in micelles, the correspond-ing mechanism might be different from the one in solution.A careful analysis of the spectral evolution of the in situ

monitored 1H NMR spectra helps us to conclude a total of fourreaction stages. What is more, the structure of the basicoligomer, an aniline tetramer, is derived. Fortunately, theintermediate in this reaction has a reasonable stability, whichallows us to characterize it fully with COSY and HSQCexperiments. (See the Supporting Information for detailedspectral analysis.)On the basis of these results, we propose the polymerization

mechanism of aniline as shown in Figure 2. The initiallyproduced free radical cations are coupled into the dimericcations, which are rearranged into Intermediate 1. Next, 1 is

stabilized with aniline stepwise to 3. One substantial differenceamong these intermediates is the varied reactivity and thereafterthe corresponding kinetics in the cluster formation. During thepropagation, the tetramer generated from 1 can react with 3 toextend the chain length. Because of the nature of thisintermediate, it can be expected that the chain-length of thepolymer is extended up to four monomer units each time,differing from the conventional ways. As a supplementary, thecations are in their singlet states, and so no paramagneticinduced line-broadening is observed.A careful comparison of the spectra for the predominant

intermediate in different SDS micelles reveals that theirstructure varies. For example, Intermediate 1 is dominant inthe reaction in spherical micelles or free from SDS, that is,[SDS] < 0.008 M, whereas 2 and 3 become predominant inrod-like micelles; moreover, the proportion of 3 increases at ahigher SDS concentration. (The corresponding spectra areshown in Figure S8 in the Supporting Information.) We believethe formation of these varied nitrenium intermediates has adirect relationship with the contribution of SDS micelles tostabilize the nascent ions. At a higher concentration of SDS, theions are more stable, thus making the formation ofintermediates 2 and 3 kinetically possible. These data suggestthat the aggregation status of the templates may have a greatimpact on the reaction pathways via controlling theintermediate structure.The nucleation proceeds after the formation of the tetramer.

The complete in situ monitored 1H NMR spectra for 0.086 Maniline in 0.10 M SDS are shown in Figure 3. The main focus ison the changes from aniline tetramers, water (HOD) as well as

Figure 1. Schematic illustration of the status of aniline upon dissolvingin (a) 0.005, (b) 0.008, (c) 0.04, (d) 0.06, and (e) 0.10 M SDSmicelles.

Figure 2. Proposed mechanism for polymerization of aniline in SDSmicelles when the reaction system is standing still.

Figure 3. In situ solution 1H NMR spectra monitoring the temporalchanges in the nucleation of polyaniline in 0.10 M SDS at 298.3 K.

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SDS, paying particular attention to the interrelationshipbetween the signal transitions from each species in timesequence.At first, a continuous downfield shift is observed for HOD

peak due to the acceptance of the protons released during theformation of aniline oligomers. Afterward, the most remarkableevent happens, in which the resonance from the water in bulkshifts to upfield continuously even though the total shift is only4.0 Hz. At the same time period, a new broad peak at 4.86 ppmgradually grows to be pronounced. More importantly, thesechanges are coincident with the appearance of the signals fromaniline tetramers. This correlation may suggest that (1) thelocal environment of the bulk water (4.83 ppm) is enrichedwith some compounds containing more electron-donatinggroups, and (2) an extra water phase has been formed.Up to now, the only new species introduced is the aniline

tetramers, possessing free amino groups, which could be theonly candidate to induce the bulk HOD upfield shifting,demonstrating the presence of tetramers in the aqueous phase.In contrast, their initial formation sites are in SDS micellebecause aniline is located in the hydrophobic core prior to thepolymerization, and that reaction has taken place within themicelle. Therefore, a translocation of the tetramer from themicelle to the aqueous phase should occur. The driving forcefor this transition is attributed to the hydrophilic nature of theprotonated oligomers, incompatible with their hydrophobiclocal environment. The surface potential of the oligomers infree states is too high to be stabilized in aqueous bulk. Mostlikely, they are protected with a layer of SDS while leaving thebulk micelles. In this way, the hydration shell from the micelleencapsulating the tetramers will form a new separate phase.Indeed, the simultaneously observed two different groups ofresonances from HOD and SDS in Figure 3 (the spectrum as ared line) support the above speculation. We assign the newpeaks at 4.86 and 4.00 ppm to the water in hydration shells andSDS head groups from the newly formed micelles encapsulatingoligomers. Because of the continuous deprotonation from theoligomers, this hydration shell is enriched with protons.After the tetramer precipitation from the bulk micelles, the

following spectral changes occur: (1) the resonance fromprotons on the ring in reduced leucoemeraldine, whoseelectron-density is affected the most in a protonated tetramer,shifts to upfield; and (2) the HOD signal has a tendency to beaveraged, until an average at 4.840 ppm is observed. Incombination, this process corresponds to a deprotonation fromthe tetramers. Theoretical studies have revealed that thefluctuation induced by either density or the wetting from waterplays an essential role in the nucleation.63,64 Upon deprotona-tion, the induced fluctuation will accelerate the micellar fusionand the regular packing between oligomers. Therefore, thedeprotonation of the tetramer functions as the real driving forcefor the nucleation to trigger the reorganization of the micellesinto the nascent nuclei. Thus, initiated density fluctuation willaccelerate the solvent exchange, leading an averaged HODsignal is observed finally. Modulating and designing strategiesto control the ways, the degrees, as well as the kineticproperties of the deprotonation is of fundamental importanceto the nucleation. Although many researchers are unaware ofthis, they have tried to achieve this goal in different ways viaapproaches such as pH attenuation,65 employment of differentdoping acid,66,67 and modulation of hetero- and homophasenucleation via approaches such as seeded,68,69 dilution,70,71 orinterfacial polymerization.72 Combining with the observation

that some structurally regular tetramers enable one to assembleinto multiple well-defined nanostructures,73 we conclude thattetramers are the actual nucleation agents in this process andthat the co-operation of the tetramer deprotonation and thefusion of the protective SDS micelles guarantees the steadynucleation. Our NMR results on the nucleation of polyanilinein the rod-like micelles are illustrated in Figure 4.

More interestingly, the signal evolution from HOD providesa wealth of information on the tetramer aggregation, the keystep in the nucleation. First, the chemical shift differencebetween the water in the hydration shell from the micellesencapsulating with oligomers and that in bulk (Δδ) originatesfrom the tetramer deprotonation, associating with the amountof tetramers integrated. In general, more tetramers are requiredto achieve a much downfielded signal, and thus a larger size ofthe nucleus phase. Therefore, the corresponding nuclei sizeincreases with an increment of SDS concentration (theobserved Δδ is respectively 3.98, 7.06, and 11.66 Hz in 0.04,0.06, and 0.10 M SDS). The classical nucleation theory hasdemonstrated the nucleus size largely determines the height ofthe free energy barrier for nucleation and hence the nucleationrate. A nucleus with larger size has to overcome a higher energybarrier, inducing a slower nucleation rate,74,75 and therefore aproduct with smaller size will be generated in a fixed growthperiod. As shown in Figure 5, the TEM images for thenanofibers display an average diameter of 80, 60, and 20 nm,respectively, agreeing quite well with the results derived fromthe chemical shift difference, demonstrating the effectiveness ofusing the latter to evaluate the nuclei sizes. Second, thetemporal change of this signal reflects the structural growthbehavior in the nucleation. For the polymerization in 0.06 and0.10 M SDS, only one HOD averaging process is observed. Incontrast, such process happens twice as deduced from the twoaveraged peaks at 4.850 and 4.855 ppm in 0.04 M SDS. Becauseone cycle of such signal averaging indicates one completenucleation, the spectral pattern in 0.04 M SDS indicates thepresence of the secondary nucleation. If this occurs, a lessordered structure will be produced. TEM images at highmagnification (Figure 5a) show that the borderline of eachnanofiber in polyaniline prepared from 0.04 M SDS is indeednot as regular as those obtained from the other two. Third, tworesonances from the water at 4.86 and 4.87 ppm are detected in0.06 M in the signal averaging. Because these signals have a

Figure 4. Schematic representation of the nucleation for polyaniline inSDS rod-like micelles.

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distinct correlation with the property fluctuation as induced bythe deprotonation of the tetramers, the emerging of multiplepeaks indicates the presence of more than one type of thenucleation, leading to the nascent nuclei with varied sizes. TEMimages confirm that the diameter size for the sample from 0.06M SDS has a wide distribution. Hence, the nucleationinformation has been encoded in the process of solventexchange between the solvent in solvation shell and that inbulk, making solvent a sensitive probe to decode this invisibleprocess. Any successful spectroscopic attempts to establish itscorrelation with the nucleation might provide valuableinformation that cannot be obtained otherwise.Because the status of all species differs significantly in

spherical and rod-like micelles, the in situ 1H NMRmeasurements were carried out in the spherical systems aswell. Figure 6 displays the recorded spectra for the polymer-

ization of 0.086 M aniline in 0.005 M SDS. In great contrast,this time the initial pronounced spectral changes of the reactionenvironment occur in the SDS part rather than in the HOD. Asthe reaction goes, the line-shapes of the resonances from SDSchange continuously over time, but the corresponding chemicalshifts remain the same. Specifically, the line-shape from α-CH2is broadened and intensities are weakened, and finallyimmersed into the baseline, while that from β- and γ-CH2turns into an asymmetric shape at first and then becomesessentially symmetric again. The spectral change of HOD asobserved in rod-like systems takes place when the abovetransition is over. This fact suggests a structural transition hastaken place in micelles prior to the nucleation.

Hassan and coauthors have demonstrated that aniliniumchloride impacts a profound effect on the size and shape of SDSmicelles by using dynamic and static light scatteringtechniques.76 Additionally, a number of systematic studiesfrom SANS, SAXS, and 1H NMR have demonstrated thatadsorption of electrolytes such as inorganic or organic salts,some effective interfacial curvature-control reagents, on ionicmicelles will induce their morphological changes by screeningthe electrostatic interactions.77−79 The induction of thismicellar transitions has a critical correlation with the locationand charge of these electrolytes. All of these criteria are methere; that is, the continuously in situ formed anilinium at SDShead will induce the structural transition of the micelle.Therefore, the asymmetric line-shapes from β- and γ-CH2

reflecting the chemical shift anisotropy properties of thesegroups can be assigned to a micelle with ellipsoidal structure,whereas the final structure is a circular bilayer-like one asevidenced by the disappearance of the α-CH2 peak due to thepresence of strong dipolar interactions with an increment of thegauche conformers in SDS and the symmetric broad peaksfrom β- and γ-CH2. Further nucleation process followsprocesses similar to those observed in the rod-like systems.Immersing in such bilayered micelles, the parallel mergingbetween the tetramers is favorable, and hence, as demonstratedby TEM (Figure S9 in the Supporting Information), only flatplate-like nanostructures are produced. The correspondingnucleation mechanism of polyaniline in SDS spherical micellesis displayed in Figure 7. Our experiment demonstrates that theassembly profile of the templates can be modified in the

Figure 5. TEM images and 1H NMR signal evolution from HOD for polyaniline prepared from (a) 0.04, (b) 0.06, and (c) 0.10 M SDS at 298.3 K.Scale bar = 2.0 μm (200 nm for the inset).

Figure 6. In situ 1H NMR spectra of the polymerization of 0.086 Maniline in 0.005 M of SDS as a function of time.

Figure 7. Schematic illustration of the nucleation of polyaniline in SDSmicelles.

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reaction, and the morphology of the polymer is determined bythe template structure in the nucleation.

■ CONCLUSIONS

A mechanistic study on the nucleation of nanomaterials fromSDS micelles, focusing on the transition from the clusters to thenuclei, has been performed using polyaniline as a modelcompound. An unconventional cationic chain polymerizationproceeding through a nitrenium intermediate is proposed.Further NMR signal evolution from SDS, HOD, and thetetramers allows us to conclude the nucleation proceeds withthe translocation of the protonated tetramers from the micelleto the aqueous bulk, deprotonation of the oligomers to inducethe micellar fusion, and intermolecular packing to form nascentnuclei. The steady progress of the nucleation is guaranteed bythe formation of an SDS protective outlayer. More importantly,our work demonstrates that the utilization of the solvent signalin solvation shell is a sensitive approach to reveal the nucleationprocess and provides sufficient information on the nuclei sizeand growth dynamics. Finally, anilinium and other protonatedoligomers produced in situ have triggered a structural transitionin spherical micelles before the nucleation. The morphology ofthe polymer depends critically on the structure of the micelle inthe nucleation. This work initiates the mechanistic study on thenucleation of nanomaterials, in particular polymeric nano-particles, at the molecular scale.

■ ASSOCIATED CONTENT

*S Supporting InformationNMR spectroscopic analyses on the solubilization locus ofaniline before polymerization and the spectral changes in thecourse of the reaction, the structural characterization of themost stable intermediate (intermediate 3), the calculatedstructure of intermediate 3 from theoretical simulation, and acomparison of the 1H NMR spectra for the intermediatesproduced in the polymerization at different concentrations ofSDS. This material is available free of charge via the Internet athttp://pubs.acs.org.

■ AUTHOR INFORMATION

Corresponding Author*Phone: +86 25 84315943. E-mail: wuxd@mail.njust.edu.cn,wxduml@yahoo.com.

NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTS

We thank Drs. Xiaohong Jiang, Xiaoheng Liu, and Junwu Zhufor their helpful discussions. X.W. appreciates Drs. Peng Xu,Riqiang Fu, Kun Si, and Ryszard Wycisk for reviewing thewhole manuscript and providing valuable suggestions. Wewould like to acknowledge financial support from the NaturalScience Foundation of China (grant no. 20974045), theNatural Science Foundation of Jiangsu Province (no.BK2009385), and the NUST Research Funding (grant no.2011ZDJH03).

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The Journal of Physical Chemistry C Article

dx.doi.org/10.1021/jp312803x | J. Phys. Chem. C 2013, 117, 9477−94849484